Welding and plasma arc torches are widely used in the welding, cutting, and marking of materials. A plasma torch generally includes an electrode and a nozzle having a central exit orifice mounted within a torch body, electrical connections, passages for cooling, passages for arc control fluids (e.g., plasma gas), and a power supply. Optionally, a swirl ring is employed to control fluid flow patterns in the plasma chamber formed between the electrode and nozzle. The torch produces a plasma arc, a constricted ionized jet of a gas with high temperature and high momentum. Gases used in the torch can be non-reactive (e.g., argon or nitrogen) or reactive (e.g., oxygen or air). In operation, a pilot arc is first generated between the electrode (cathode) and the nozzle (anode). Generation of the pilot arc can be by means of a high frequency, high voltage signal coupled to a DC power supply and the torch or by means of any of a variety of contact starting methods.
In some torches, a shield is used to prevent molten spatter from damaging the other components of the torch, for example, the electrode, nozzle, or swirl ring. Often, the molten spatter builds up on the shield causing double arcing or melting of the shield. The build-up typically increases as the cutting time increases.
To decrease the amount of molten spatter that builds up on the shield, prior torches have used shields with vent slots added to the end face of the shield. The vent slots act as channels for melted metal, for example, slag, to leave the end face of the shield. These shields employ four slots that are arranged in a symmetrical pattern about the end face surrounding a plasma exit orifice. The slots retain the same dimensions as a function of distance from the plasma exit orifice.
Prior art shields with vent slots do not adequately remove slag for certain applications. Instead, the slag builds up on the shield and within the vent slots. The slag can block all or a portion of the vent slots resulting in a double arc or melting of the shield. Cleaning slag build-up is difficult. Operators often replace the shield once build-up has occurred instead of performing the time-consuming task of cleaning the shield. The slag build-up can also lead to premature failure of the shield and, sometimes, the premature failure of other consumables. In addition, slag build-up can increase the down time of the torch because the operator is required to stop the system to either clean or replace the shield when too much build-up occurs. Increased down time and premature consumable failure result in increased operating costs.